The present invention relates to a thermal transfer sheet and more particularly to a thermal transfer image-receiving sheet capable of forming a record image excellent in color density, sharpness and various types of fastness, particularly durability such as light fastness.
Various thermal transfer processes are known in the art. One of them is a transfer process which comprises supporting a sublimable dye as a recording agent on a substrate sheet, such as a polyester film, to form a thermal transfer sheet and forming various full color images on an image-receiving sheet dyeable with a sublimable dye, for example, an image-receiving sheet comprising paper, a plastic film or the like and, formed thereon, a dye-receiving layer.
In this case, a thermal head of a printer is used as heating means, and a number of color dots of three or four colors are transferred to the image-receiving material, thereby reproducing a full color image of an original by means of the multicolor dots.
Since the color material used is a dye, the image thus formed is very clear and highly transparent, so that the resultant image is excellent in reproducibility and gradation; and the quality of the image is the same as that of an image formed by conventional offset printing and gravure printing. In this method, it is possible to form an image having a high quality comparable to a full color photographic image.
Since, however, the resultant image comprises a dye, the light fastness is generally inferior to that of an image comprising a pigment, so that the image rapidly fades or discolors when it is exposed to direct sunlight.
In order to solve the above-described drawbacks, Japanese Patent Laid-Open Publication Nos. 101090/1985, 130735/1985, 54982/1986, 229594/1986 and 141287/1990 disclose a technique wherein ultraviolet absorber or an antioxidant is incorporated in a dye-receiving layer of the thermal transfer image-receiving sheet.
The addition of the ultraviolet absorber can improve the light fastness to some extent. The-method wherein the ultraviolet absorber is merely incorporated in the dye-receiving layer Gives rise to a problem that the ultraviolet absorber bleeds out on the surface of the dye receiving layer and disappears or evaporates or decomposes when it is exposed to heat, so that the effect of the ultraviolet absorbers decreases with the elapse of time.
The fading of the dye image is attributable to an incident ultraviolet radiation and further accelerated also by an ultraviolet radiation which passes through a dye receiving layer, reaches the substrate sheet, reflects from the surface of the substrate sheet and again scatters in the dye-receiving layer. The above-described fading derived from the reflected light from the substrate sheet cannot be prevented by a simple method wherein an ultraviolet absorber is added on the dye-receiving layer or incorporated in the dye-receiving layer.
In particular, if the substrate sheet of the thermal transfer sheet is a white sheet, such as paper, there is a limitation on the effect when an ultraviolet absorber is incorporated in the dye receiving layer. Studies conducted by the present inventors have revealed that the ultraviolet radiation passed through the dye-receiving layer reflects again from the surface of the white substrate sheet and the reflected ultraviolet radiation irregularly reflects within the receiving layer to lower the light fastness of the image.